This invention relates to contact lenses and to a method for their preparation. More particularly, this invention relates to a method of applying tint or other dyestuff to a hydrophilic contact lens while the lens is in its xerogel, or "dry," state.
Tinted or otherwise dyed contact lenses are used as means for cosmetically altering eye color, as well as for providing means for aiding in the visualization of the lens itself. A problem exists, however, in that many of the known methods for tinting or otherwise dying plastic materials are unsuitable for practical coloring of hydrophilic (commonly referred to as "soft") contact lenses. For example, the blending of dye into the lens material is not useful because the presence of water in the hydrophilic polymer encourages migration and leaching of the dye through and out of the lens body. Therefore, it has become desirable in the industry to utilize a method which involves placing the dye on an outer surface of an otherwise standard, clear ("untinted") lens.
A number of techniques have been used to place colorant on the outside surface of a clear lens. One approach has been the use of printing, transfer or coating methods. According to this process, a coloring agent is caused to adhere to the surface of the plastic lens. However, this process is not well suited for coloring water-absorbable materials as used in soft contact lenses. First, there exists a tendency for the dye to form blotches towards the center of the lens, particularly in the area of the lens corresponding to the pupil of the wearer. Also, if a dry hydrophilic lens colored by this method is allowed to swell in water, the applied layer of coloring agent does not swell as much as the plastic itself. Consequently, the coloring agent will become dissociated from the surface of the lens, thereby creating deformation and cosmetic defects in the product. One method of overcoming this problem has been to use standard dot matrix printing techniques, thereby allowing spaces between the dots which allow for swelling of the lens. However, while such a method may allow printing on a dry lens, the resulting product does not have a solid covering of tint and is therefore artificial looking.
One method of providing a solid cover of color on a clear lens without deformation has been to apply the colorant while the lens is in its hydrated state. In this method, a hydrated lens is placed into a fixture having three interlocking pieces. A bottom piece is a dome upon which the lens is positioned, a top piece rests over the lens and acts as a reservoir for the dye solution, and a third piece for housing the top and bottom pieces. The entire fixture is then inserted into a clamp, and the dye solution is placed into the reservoir. The dye solution is allowed to remain in contact with the lens until the desired shade is obtained. The reservoir is then purged with water until all traces of the dye solution have been removed. The lens is then removed from the fixture. This method is disclosed in U.S. Pat. No. 4,559,058 to Su. This method is problematic, however, in that it is labor and space intensive, wastes dye solution and requires a large amount of purging water.
There exists a need, therefore, for a method of providing a solid cover of dye onto a contact lens which can be performed when the lens is in its xerogel state.
There also exists a need for such a method which eliminates the problem of blotching of dye at the center of the lens.
There is a further need for such a method which is inexpensive, clean and environmentally efficient.